Re: [PATCH v1 1/2] LoongArch: BPF: Make arena support depend on SCQ extension

[Tiezhu Yang]

On 2026/11/2 下午10:20, Huacai Chen wrote:
> Hi, Tiezhu,
>
> On Mon, Nov 2, 2026 at 3:28 PM Tiezhu Yang <yangtiezhu@xxxxxxxxxxx> wrote:
> > When testing bpf selftest "jit_insns/cast_to_user", there are different
> > behaviors on different Loongson machines.

...

> > diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
> > index 3ab039d0afb6..01c12da168ee 100644
> > --- a/arch/loongarch/net/bpf_jit.c
> > +++ b/arch/loongarch/net/bpf_jit.c
> > @@ -2489,7 +2489,15 @@ bool bpf_jit_bypass_spec_v4(void)
> >
> >   bool bpf_jit_supports_arena(void)
> >   {
> > -       return true;
> > +       /*
> > +        * The arena range tree uses kmalloc_nolock(), which requires 128-bit
> > +        * atomic cmpxchg support, provided by SCQ extension on LoongArch.
> > +        */
> > +#ifdef system_has_cmpxchg128
> > +       return system_has_cmpxchg128();
> > +#else
> > +       return false;
> > +#endif
> You can "return cpu_has_scq" directly.

This was my initial thought too; it is simpler, but unfortunately
not enough here.

The 128-bit atomic cmpxchg arch_cmpxchg128() not only depends on
the hardware SCQ extension, but also requires assembler support,
which was addressed in commit c8b8f3c50f48 ("LoongArch: Only use
SC.Q when supported by the assembler").

Therefore, using #ifdef system_has_cmpxchg128 as in the current
patch is necessary to handle toolchain compatibility properly.

Here is a simple diff to verify this scenario. On a 3A6000 machine
with hardware SCQ extension, if we hack bpf_jit_supports_arena() to
return cpu_has_scq directly, but force AS_HAS_SCQ_EXTENSION to 'n'
to simulate an environment without assembler support, the original
test error "libbpf: map 'arena': failed to create: -ENOMEM" still
shows up because the underlying slab allocator lacks the flag
__CMPXCHG_DOUBLE.

```
diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index b5ed4881d3c0..6f572d24d2c3 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -336,7 +336,7 @@ config AS_HAS_LVZ_EXTENSION
        def_bool $(as-instr,hvcl 0)

 config AS_HAS_SCQ_EXTENSION
-       def_bool $(as-instr,sc.q \$t0$(comma)\$t1$(comma)\$t2)
+       def_bool n

 config CC_HAS_ANNOTATE_TABLEJUMP
        def_bool $(cc-option,-mannotate-tablejump)
diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index 8cc78efe722d..1e8049937851 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -2490,15 +2490,7 @@ bool bpf_jit_bypass_spec_v4(void)

 bool bpf_jit_supports_arena(void)
 {
-       /*
-        * The arena range tree uses kmalloc_nolock(), which requires 
128-bit
-        * atomic cmpxchg support, provided by SCQ extension on LoongArch.
-        */
-#ifdef system_has_cmpxchg128
-       return system_has_cmpxchg128();
-#else
-       return false;
-#endif
+       return cpu_has_scq;
 }

 bool bpf_jit_supports_fsession(void)
```

So, the current patch has no problem IMO.

Thanks,
Tiezhu